The prior art discloses multiple disc wet brakes that are actuated by an annular hydraulic piston for normal service braking of an output shaft on a transmission of an off-highway implement. In such devices, hydraulic pressure is applied to the piston thereby forcing rotating brake discs that turn with the transmission output shaft into braking engagement with stationary brake discs thereby applying a braking action to the shaft. The prior art also discloses hydraulic type brakes that employ springs for forcing the brake discs or members into engagement relative to each other.
U.S. Pat. No. 3,974,896 to D. T. Rach discloses a brake assembly having a hydraulically actuated service brake and an independently spring applied park brake. The Rach brake design provides for emergency braking in case of hydraulic failure in the service brake. The Rach design further discloses springs for relieving pressure on the brake when no hydraulic pressure is available to apply to the service brake. As in most hydraulic brake designs, the brake piston in the Rach brake assembly is moved between a released position, wherein the brake piston applies no pressure to the annular brake discs, and an engaged position, wherein the piston applies a compressive force to the annular brake discs thereby applying a braking action to the output shaft of the transmission. During operation of the brake, an increment of wear will take place on the brake surfaces provided on opposite sides of each annular brake disc. As a result of the wear on the discs, there is an increase in the distance the brake piston travels between released and engaged positions.
To optimize the brake drag between the annular discs and to provide a substantially constant brake actuation response, it is important that the distance the brake piston travels between released and engaged positions be accurately controlled. As will be appreciated, the larger the clearance between the released position of the brake piston and the annular brake discs, the lower will be the brake drag developed between the annular brake discs and a longer response time will be required to brake the transmission output shaft. On the other hand, a shorter response time to actuate the brake can be effected by providing a smaller clearance between the released position of the brake piston and the annular brake discs. This reduced amount of clearance between the released position of the brake piston and the annular brake discs, however, will increase the brake drag thereby requiting more energy to operate the agricultural implement.
As will be appreciated, the brake drag between the annular brake discs should be maintained as low as possible when the brake piston is in a released position and the response to actuate the brake should be as fast as possible. A compromise between the degree of brake drag and the response time of the brake assembly is established when the brake assembly is designed and initially assembled. Inherent wear of the friction surfaces on the brake discs, however, changes the performance of the brake assembly over time and can lead to serious problems due to unpredictability of the brake assembly during operation of the agricultural implement.
Thus, there is a need and a desire for a wet disc brake assembly including a brake release mechanism that is self compensating to correct for the wear between the brake discs thus yielding substantially constant the brake operation during the useful life of brake assembly.